Advantages of patterning magnetic materials and methods of patterning magnetic layers are known. Typically, patterning is carried out using techniques common in the art of semiconductor processing, such as photolithographic techniques.
One method of photolithographically patterning a homogeneously deposited magnetic film is to coat the film with a photoresist and then to remove selected portions of the photoresist in order to expose regions of the magnetic film. The exposed regions of the film may then be removed by applying an etchant. The remaining photoresist is stripped, leaving the magnetic film in a desired pattern.
Magnetic film patterning can also be accomplished during the step of depositing the film on a substrate. For example, during a sputter deposition, a mask can be positioned between a deposition source and a substrate on which the film is to be deposited. The mask is patterned to selectively block or pass the magnetic material being deposited onto the substrate. This technique is employed with acceptable results for patterns that need not be precise, since vapor scattering which occurs as the deposition material travels through the mask will limit the preciseness of patterns formed in this manner.
U.S. Pat. No. 5,017,255 to Calhoun et al. describes a method of transferring a pattern onto a substrate. The substrate is made of an embossable material, and is typically a polymer such as polyimide. The embossed polymeric substrate can be formed by extruding a softened polymer onto a machined embossing roll having an imprinted negative of a desired pattern, so as to imprint the design into the polymer. Calhoun et al. teaches that typically a metal layer is then deposited onto the embossed surface of the polymeric substrate, but semiconductor or dielectric materials may also be employed. Calhoun et al. does not describe the method as being employed with magnetic materials.
An ever-present goal in the process of fabricating magnetic devices is maximizing the magnetic properties of the magnetic structure for its intended purpose. Some devices require one or more layers of magnetically "soft" material, i.e. a material having a high permeability and a low coercive force. Other devices include one or more magnetically "hard" layers, i.e. layers having a high coercive force. Still other devices include a combination of magnetically hard and magnetically soft layers. For example, magnetoresistive recording heads utilize patterned hard magnetic materials to bias soft magnetic films in order to obtain optimum performance. The heads operate best when the magnetization vector is at an angle of approximately 45.degree. to the direction of flux travel. However, magnetoresistive heads are typically rectangular, so that the demagnetizing field aligns the magnetization vector at an angle of 90.degree. to the direction of flux travel. Therefore, a magnetically hard film that has a high level of stray flux is placed adjacent to the magnetically soft magnetoresistive head to rotate, or bias, the magnetization vector into the position for optimum performance. Because precise control of the biasing field is necessary in order to properly bias the head, the magnetically hard film is patterned to form a structure that provides the desired control.
Low frequency electromagnetic interference (EMI) shielding is another application for patterned magnetic layers. The shielding properties of a given material are proportional to the product of permeability and electrical conductivity, and are inversely proportional to the frequency of interest. Therefore, while high frequency shields can be made with thin films of conductive metal, low frequency metal films are generally quite thick. However, magnetic materials such as supermalloy or permalloy having a high permeability and electrical conductivity can be used to form thin low frequency EMI shields. The materials can be placed on containers enclosing sensitive electronic equipment, such as computers. Maximum EMI protection is obtained by causing the shield to be absorbent of the EMI, rather than being reflective. However, absorption is hindered by eddy currents which prevent the EMI from entering the energy-absorbent material. For thin films, eddy currents are proportional to conductivity, permeability, frequency and surface area. For fixed material properties, eddy current shielding can be reduced by decreasing the effective surface area of the EMI-absorbing material. A homogeneous film of metallic magnetic material can be patterned to form an array of separate, electrically isolated regions spaced apart by regions in which the material has been removed.
Another structure that is formed using patterned magnetic layers is a magnetic electronic article surveillance (EAS) tag, as described in U.S. Pat. Nos. 4,960,651 to Pettigrew et al. and 5,083,122 to Piotrowski et al. EAS tags are also referred to as anti-pilferage tags, and consist of one or more high permeability, low coercive force magnetic films. If more than one soft magnetic thin film is used, adjacent films are separated by a non-magnetic thin film. An EAS tag that is passed through an interrogating magnetic field which exceeds the coercive force of the tag will undergo changes in the direction of the magnetization vector of the tag in response to the interrogating field. The orientation changes of the magnetization vector are measurable as a voltage signal by detector electronics of a surveillance system. In addition to the magnetically soft films, a layer of magnetically hard material may be employed to allow the tag to be selectively deactivated. Deactivation is accomplished by placing the magnetically hard layer in its fully magnetized state. In this state, the fringing field from the hard magnetic layer penetrates the soft magnetic layer or layers, preventing the tag from responding to the interrogating field.
As previously noted, the operation of a magneto-resistive recording head, an EMI shield and an EAS tag depends to a significant degree upon fabrication techniques utilized in forming the structure. An object of the invention is to provide a method of patterning magnetic material, wherein desired magnetic properties are more easily achieved than by employing the approaches described above.